The present invention relates in a general manner to geophysical data acquisition operations at sea using underwater seismic measurements.
1. Field of the Invention
More precisely, the invention relates to an original procedure for determining the ocean current, through means on board a data acquisition ship. The invention also relates to a device for the implementation of such a method.
2. Description of the Related Art
In order to successfully conduct geophysical data acquisition operations at sea, it is necessary to have access to measurements of ocean current. These measurements make it possible to take into account the effect of the current on the ship, and on the towed devices (in particular, hydrophones integrated into cables called “streamers” to use the widespread terminology).
To measure the current, it is known for acquisition means such as current meter buoys to be deployed in situ.
However, such a solution is hardly satisfactory insofar as it is particularly cumbersome and expensive to implement.
It is also possible to measure the relative longitudinal current with respect to the boat relatively simply by virtue of an on-board log. However, such a log provides only a unidirectional measurement, relative to the ship.
Another on-board solution for measuring current consists in utilizing the measurements of a hull profiler of the Acoustic Doppler Current Profiler (ADCP) type to use the widespread terminology.
Such a profiler comprises means for transmitting acoustic beams into the water in several directions, the transmission means being fixed under the hull of the ship, and means for receiving and analyzing the signals reflected by the water particles.
As a function of the frequency variations observed for the various signals received, it is possible to determine the current at various depths.
The hull profiler thus makes it possible to determine a flow current with respect to the ship, from which the ship's forward vector is subsequently deducted in order to obtain the absolute current.
However, several drawbacks are associated with a hull profiler as envisaged hereinabove.
A first drawback is that it is necessary to carry out very fine adjustment (of the order of 1/10th of a degree) of the orientation of the device with respect to the ship, in order to avoid biases in the measured current vectors.
A second drawback of hull profilers is that these devices generate measurement errors when the ship is turning.
A third drawback of profilers is related to the “ringing” phenomenon. This phenomenon corresponds to a reverberation of the waves emitted on hull elements of the ship, the ADCP often being mounted in a hull-related structure, such as a well in the hull of the ship.
Such reverberation produces reflected waves which are received by the device as waves representative of a zero relative current (since it has the same frequency as the waves emitted). This disturbance may have an appreciable effect on the measurements performed at small depths.
Also, this drawback may be particularly penalizing in the context of seismic measurements at sea, insofar as for ships with large draft, “small depths” with respect to the device fixed under the hull correspond substantially to the depth of submersion of the streamers, so that the measurement of current at these depths is not reliable.